Field of the Invention
The present invention relates to a method of quantifying hydrocarbon formation and retention within a macromolecular chemical system.
Description of the Prior Art
The following documents, mentioned in the description hereafter, illustrate the state of the art:                Behar F., Vandenbroucke M., Tang Y., Marquis F., Espitalié J., 1997. Thermal Cracking of Kerogen in Open and Closed Systems: Determination of Kinetic Parameters and Stoichiometric Coefficients for Oil and Gas Generation. Org. Geochem., 26, 5-6, 321-339.        Burnham, A. K. and Braun, R. L., 1989. Development of Detailed Model of Petroleum Formation, Destruction, and Expulsion from Lacustrine and Marine Source Rocks. Advances in Organic Geochemisrty, 16, 1-3, 27-39.        Burnham, A. K. and Braun, R. L., 1990. Mathematical Model of Oil Generation, Degradation, and Expulsion. Energy and Fuel, 4, 132-146.        Faulon, J. L., Prediction Elucidation and Molecular Modeling. Algorithms and Applications in Geochemistry, Ph. D. Thesis, Edited by Ecole des Mines, Paris, 1991.        Faulon, J. L., Stochastic Generator of Chemical Structure (4) Building Polymeric Systems with Specified Properties, J. Comput. Chem., 2001, 22, 580-590.        Freund, H., Walters, C. C., Kelemen, S. R., Siskin, M., Curry, D. J., Xiao, Y., Olmstead, W. N., Gorbaty, M. L., Bence, A. E., 2005. Predicting Oil and Gas Compositional Yields via Chemical Structure-Chemical Yield Modeling (CS-CYM). Organic Geochemistry: Challenges for the 21st Century (Vol. 1), 22 IMOG Seville, Spain, 66-67.        Hatcher, P. G., 1988. Dipolar-Dephasing 13C Studies of Decomposed Wood and Coalified Xylem Tissue: Evidence for Chemical Structural Changes Associated with Defunctionalization of Lignin Structual Units During Coalification. Energy & Fuels 2, 48-58.        Pepper, A. S., 1991. Estimating the Petroleum Expulsion Behaviour of Source Rocks: A Novel Quantitative Approach. In Petroleum Migration (Edited by England W. A. and Feed A. J.), Geological Society, Special Publication. 59, pp. 9-31.        Pepper, A. S., Corvi, P. J., 1995. Simple Kinetic Models of Petroleum Formation. PartIII: Modelling and Open system. Marin and Petroleum Geology, 12, 4, 417-452.        Pepper, A. S., Dodd, T. A., 1995. Simple Models of Petroleum Formation. Part II: Oil to GAS cracking. Mar. Petrol. Geol., 12, 321-340.        Ritter, U., 2003. Fractionation of Petroleum During Expulsion fromKerogen. Journal of Geochemical Exploration 0.78-79, 417-420.        Ritter, U., 2003. Solubility of Petroleum Compounds inKerogen: Implications for Petroleum Expulsion. Organic Geochemistry 0.34, 319-326.        Tissot, B. 1969. Revue Inst. Fr. Pétrole, 24(4), 470-501.        Ungerer, P., 1989. State of the Art of Research in Kinetic Modelling of Oil Formation andExplusion. In Advances in Organic Goechemistry, Organic Geochemistry. 16, 1-3, 1-25.        Van Duin A. C. T, Siddharth D., Lorant F., Goddard III W. A. -2001. ReaxFF: A Reactive Force Field forHydrocarbons. J. Phys. Chem. A, 0.105, 9396-9409.        
The insoluble organic material of the source rock, also referred to as kerogen, is a mixture of bio-organic macromolecules (notably biogeopolymers) having aliphatic and aromatic chemical structures that evolve in the course of geologic times with the temperature and the pressure. Thermal maturation of the kerogen in the source rock occurs through the agency of two main phenomena:                the first one is thermal cracking of the organic matter at the origin of hydrocarbons; it takes place naturally in sedimentary basins, generally at a temperature ranging between 80° C. and 200° C., and a pressure ranging from 200 to 1000 bar,        the second one is the physicochemical evolution of the petroleum products within the source rock that explains the retention and expulsion of hydrocarbons from the source rock.        
These two phenomena develop within the same context and they are juxtaposed.
In basin modelling, it is important to be able to simultaneously calibrate the amounts of hydrocarbons formed and the amounts of “free” hydrocarbons that can be expelled from the source rock and migrate to the reservoir. The process of hydrocarbon retention in kerogen is a mechanism that controls the free hydrocarbons/expelled hydrocarbons ratio. If the retention of hydrocarbons is considered to predominantly occurs in the organic matter of the source rock, this retention depends on the composition of the fluids generated, on the retention capacities of the kerogen and on the volume ratio of the solid organic matter to the liquid hydrocarbons. It is thus directly linked with the physicochemical nature of the kerogen and with the conversion ratio (Pepper, 1991).
Kerogen cracking and retention of the products from this reaction being interdependent phenomena that evolve with the thermal maturation of kerogen, it appears necessary to develop a fine analysis of the cracking reaction coupled with the retention of the products formed in the kerogen in order to be able to estimate the hydrocarbons retained in the kerogen and the hydrocarbons available for storage in reservoirs.
There are known hydrocarbon quantification models that account for either the thermal cracking reaction, or the hydrocarbon retention phenomenon, but rarely of both phenomena simultaneously:                empirical and mechanistic cracking models are used to quantify the hydrocarbons formed in the source rock,        models of hydrocarbon retention in the source rock try to explain the segregation of the hydrocarbons when they are expelled from the source rock.        
Thermal Cracking Models
Two cracking reaction modelling methods are provided in the literature: empirical models and mechanistic models.
Empirical models are based on experiments to establish the global stoichiometric equations that account for the mass balances observed.
These stoichiometric equations are coupled with hydrocarbon formation velocity laws, they correspond to a series of simultaneous, independent and competitive reactions and they were developed assuming that the global evolution of the petroleum potential of a kerogen under maturation is an irreversible kinetic process (Pepper and Dodd, 1995). The kinetic (E: activation energy, A: frequency factor) and stoichiometric (Xi: relative contribution of reaction i) parameters have to be calibrated individually because the source rocks do not generate hydrocarbons at the same rate. Artificial maturation experiments are therefore carried out in the laboratory under controlled thermal conditions. These experiments are performed on kerogens or source rocks and they can be of different natures. By numerical inversion of the laboratory data, it is possible to calculate the kinetic and stoichiometric parameters. These parameters, obtained at high temperature (300° C.-600° C.) over short times (some minutes to some days) are then assumed to be extrapolatable for lower temperatures than those of the experimental conditions such as those imposed by the geothermal gradients.
This method is currently the only means allowing providing information on the formation of hydrocarbons compatible with basin models. However, this procedure is based on many approximations. In fact, Behar et al.'s work of 1997 showed that the differences in the experimental conditions of the pyrolyses carried out (open or closed medium, in the presence or absence of water or of mineral matrix, or according to the grain size of the sample) leads to a kinetic parameters lag and therefore to an uncertainty in the estimation of the oil window. Similarly, extrapolation of the kinetic parameters at low temperature involves the nature of the cracking mechanism does not significantly develop as a function of temperature.
Mechanistic models are not based on stoichiometric equations but on elementary (radical) reactions to simulate the thermal degradation of complex macromolecules and to reproduce the distribution of the hydrocarbons formed (Freund et al., 2005). Elaboration of these models starts by modelling the initial macromolecule. This modelling is constrained by experimental data relative to the structural properties of the sample. It is based on the distribution of the functional groups in the molecule for establishing the probable structure thereof. Once the initial macromolecule is defined, elementary reactions are applied to the structure so as to simulate the formation of the thermal degradation products. Each elementary reaction has its own kinetic properties, valid whatever the temperature scale. It is thus possible to simulate thermal maturation both under laboratory conditions and under geologic conditions.
The advantage of this approach is that it minimizes the uncertainty on the reaction velocities extrapolated to the geologic conditions. On the other hand, the complexity of these models makes elaboration of the reaction mechanism difficult. Finally, and above all, the very large number of reactions in these models is incompatible with current basin simulators.
Retention Models
Physical models have been provided in order to estimate the proportion of expelled hydrocarbons as a function of the kerogen conversion rate. Ungerer's expulsion model (1989) sets a threshold corresponding to a conversion rate for which the hydrocarbons formed in the source rock are expelled. In this connection, Pepper (1991) considers correlating “the petroleum expulsion efficiency” (PPE) with the initial petroleum potential of the source rocks considered. These models reproduce more or less accurately (according to the source rock type) the quality of the hydrocarbons of the reservoir. They consider the source rock in two states only, before and after expulsion of the hydrocarbons, without taking into account the qualitative evolution of the source rock or the expulsion kinematics.
Later, Ritter (2003) provided a retention model based on the solubility of the hydrocarbons in kerogen. He established an empirical relation between the swelling ratio and the Hildebrand solubility parameter, for each type of organic matter. This relation defines a retention ratio for each group of compounds. Finally, this model confirms the fractionation sequence observed in nature, except for branched aliphatic hydrocarbons. The polymer solubility theory and this model thus do not totally explain the hydrocarbon composition differences between source rock extracts and reservoir oils, observed in petroleum systems. Similarly, this model does not explain the great accumulation of aliphatic hydrocarbons in coals. This model has two limitations: the first one is that the values of the swelling ratio are filed according to their chemical class but they are not normalized. Thus, the model does not respect the mass conservation principle. This generates too high retention thresholds and the sum of the compositions is above 100%. The second drawback is due to the fact that the swelling phenomenon involves swelling of the organic matrix. Now, there is little chance that this swelling occurs in rocks subjected to high overpressures.
In conclusion, the expulsion and retention models provided to date involve possible mechanisms and they are developed with more or less assumptions, which leads to more or less realistic approaches. In fact, the structure of kerogen and the nature of the effluents vary with the source rock maturity, therefore thermal cracking and expulsion are indissociable processes.
The method according to the invention allows quantification of the formation and the retention of hydrocarbons in a source rock from a new type of simulation. This simulation type is based on a dynamic molecular modelling technique coupled with a reactive force field. As in the case of mechanistic models, this approach requires as the starting point a “molecular” representation of the structure of the kerogen. The method according to the invention does not require writing hundreds of a priori reactions: the reaction mechanism is not an input datum, it becomes a result of the dynamic simulation. As in the case of radical mechanisms, this new technique is applicable in any thermal regime.